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The service uses information from Twitter chatter to detect popular tracks as well as new artists. Users who follow musicians can see other artists those musicians follow and can listen to 30-second clips of songs by them.

Tapping the play button on an image of an artist plays a clip from one song picked to represent them. In the case of Gotye, for example, a tap plays a preview of "Eyes Wide Open." Users can tap another button that opens iTunes where they can buy the track. Or they can play full songs by signing up for a $10-a-month subscription from Spotify or Rdio.

While users can tweet what they're listening to and add a few characters of comment, they have to go back to the regular Twitter app for normal Twitter functions. However, the people you're following and who's following you are integrated across both apps.

The music app is strikingly more visually appealing than the regular Twitter app. Squares of photos of artists fill the screen and bounce around in response to swipes. The app also starts a turntable spinning with a little picture of album cover art when playing a song.

For now, the service is only available in the U.S., Canada, the U.K., Ireland, Australia and New Zealand.

In a blog post, the company said its service "will change the way people find music." It is calling the app (hash)music, following Twitter's practice of using hashtags to organize tweets around topics.

Thursday's announcement about a music service had been expected. "American Idol" host Ryan Seacrest tweeted about it last week.

As more music is sold through the recommendations of friends on social networks, observers said it's natural for Twitter to get involved.

"Social media is the current and the next frontier in terms of marketing everything," said Larry Iser, a Los Angeles music lawyer whose firm represents Justin Bieber. "One tweet from Justin Bieber can launch a new product or a new artist. It makes sense Twitter wants to come to the space and do what Myspace has been trying to do for years."

The music service's debut comes less than three months after the release of a Twitter video app called Vine that distributes six-second clips that can be played in a continuous loop.

The expansion into other forms of media besides text and photos is part of Twitter's effort to make its messaging service even more appealing to its more than 200 million monthly users. More frequent usage of the service creates more opportunities to show ads -- the main way that Twitter makes money.

The foray into music could open up a new channel of revenue as well. Apple Inc.'s iTunes pays partners a few cents for every song sale that is a direct result of an online referral. If Twitter's recommendations persuade enough people to buy songs after hearing excerpts, these bounties could add up.

Neither Rdio or Spotify are paying bounties to Twitter for new sign-ups, the companies confirmed.

Within a few hours of the service going live, new sign-ups and song plays on Rdio were already "spiking," said Malthe Sigurdsson, Rdio's vice president of product. "This is looking really good," he said.

As with many of the other tools that it has added since its inception seven years ago, Twitter bought the technology powering its music app from a startup. In this case, the music app is based on a concept and tools honed by We Are Hunted, which shut down its site for tracking popular music last week.

Twitter, which is based in San Francisco, is winning over advertisers, who typically must package their marketing messages in 140-character characters so that they fit seamlessly into the rest of the rapid-fire chatter flowing through users' feeds.

The company's worldwide ad revenue this year is expected to more than double to $583 million, up from $283 million last year. As a private company, Twitter doesn't disclose details about its financial performance.

That could change soon, though. There is mounting speculation that Twitter is expanding its services and selling ads more aggressively in preparation for an initial public offering of stock that could come late this year or early next year. If that were to happen, it would be the hottest technology IPO since Facebook went public nearly a year ago.

Twitter CEO Dick Costolo has repeatedly said that the company isn't under any pressure to go public because it has raised ample financing from investors, including a $400 million injection from venture capitalists in July 2011.

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